Add predefined version D_Optimized when compiling with -O

[kinke]

So that code can be aware of whether it's compiled with -O, e.g., to distinguish between debug and optimized release builds (not necessarily using -release).

Rationale: https://dlang.slack.com/archives/C1ZDHBB2S/p1655556068067789

I'd have use for it for optimized druntime/Phobos unittests with LDC as well, where I've so far had to work around benign failures with -O by either preventing optimizations for unittest blocks/test helper functions (via @optStrategy("none")), or disabling some checks altogether.

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[maxhaton]

I am basically in favour of this.

[RazvanN7]

cc @WalterBright

[kinke]

Ping @WalterBright.

[RazvanN7]

@kinke let's just move on with this. Resolve the conflicts and I'll merge this.

[kinke]

@RazvanN7: Alright, rebased and spec PR added: dlang/dlang.org#3431